Panel with paint ready surface

ABSTRACT

In a preferred embodiment, a composite panel with a smooth outer surface, ready for painting with or without addition of primer, may be created by constructing a panel layup assembly upon a mold, the panel layup assembly including a composite panel having a core and a resin formulation, and a release film between the mold and the composite panel, where a smooth release surface of the release film is in contact with the composite panel upon construction; initiating curing of the composite panel at a first temperature within a lowermost ten percent of a curing temperature range of the resin formulation; continuing curing of the composite panel at a second temperature above the lowermost ten percent of the curing temperature range; and completing curing of the composite panel at a third temperature below the second temperature.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.14/631,790, filed Feb. 25, 2015, which claims priority from U.S.Provisional Application No. 61/945,330, filed Feb. 27, 2014, thecontents of both which are incorporated by reference in their entirety.

BACKGROUND

Especially for structural materials and interior materials of aircraft,reinforced fiber composite materials are increasingly used as skins inhoneycomb sandwich panels for reduction of weight. Composite sandwichpanels formed of reinforcing fibers and a resin are widely used foraircraft, automobiles and other industrial applications because of theirstrength and weight characteristics. The details of the process by whichthese panels can be formed is described generally in U.S. Pat. No.7,186,310 entitled “Method for Forming a Honeycomb Composite SandwichPanel”, the contents of which are fully incorporated herein byreference.

One goal in the construction of composite panels is to limit anddecrease an amount of surface defects, such as pitting, telegraphing,pocks, voids, and creases, which lead to both mechanical defects as wellas costly post-production operations. During manufacturing of thepanels, thermal shock in the curing step can cause pitting and dimplingof the skin surface to the panel. This thermal shock can cause volatileorganic compounds (VOC) to release resulting in small pits on thesurface of the panel. The rapid release of the VOCs result in bubbles,which in turn lead to small pits and cavities in the surface of thepanel which must be repaired prior to painting.

Honeycomb sandwich panels are typically manufactured by laminating ahoneycomb core made of aramid paper with prepreg laminates on both sidesand curing the prepreg laminates while bonding the prepreg laminates tothe honeycomb core as so-called co-curing.

A pre-impregnated composite fabric or prepreg is a fabric reinforcementthat is pre-impregnated with a resin system. In this case, the adhesivestrength between the honeycomb core and the prepreg laminates as skinpanels is important. A method of including adhesive films between thehoneycomb core and the prepreg laminates and curing the prepreglaminates together with the adhesive films for fabricating a sandwichpanel has been used.

Furthermore, to decrease surface defects such as pits and resin blurs onthe panel's skins, one approach has been to stick adhesive films on thesurfaces of prepreg laminates, to allow them to cure together with theprepreg laminates. However, this approach increases weight, materialcost and labor cost.

In typical manufacturing operations for aerospace applications, afterassembly the panels are subjected to a “fill and fare” to addresssurface defects. That is, the manufacturing process produces a panelsurface that contains inherent and unacceptable defects such as pittingand telegraphing. Pitting leaves very small pinholes in the surface of apanel that must be filled prior to any painting. Telegraphing is aresult of the facing material drooping into the empty area of thehoneycomb core. This produces a surface that is not perfectly flat andcan be observed after the painting process unless further remediation isperformed on the finished panel.

The first step of the “fill and fare” operation is a fill and sand step,where putty material is spread onto the surface to fill in the gaps,pits, and holes. There is significant labor in this step, which addscosts to the panel, and adds a slight weight increase as well due to theadded weight of the putty material. Alternatively, surface films can beadded to provide smooth surface to the panel. However, there areincreased material costs associated with surfacing films that add weightto the panel as well as labor costs needed to apply the film. In somecases, peel ply materials are used, which also add weight and time tothe manufacturing process.

All three of these remedial steps use expendable release materials toprocess the panels. Moreover, these “fill and fare” operations are bothtime consuming and costly, and add significantly to the manufacturingcosts of the panels. It would be desirable to eliminate or significantlyreduce the amount of “fill and fare” procedures prior to painting.

SUMMARY OF ILLUSTRATIVE EMBODIMENTS

By altering the release materials and processing parameters, certainembodiments eliminate or reduce the amount of extra labor and materialcosts associated with a fill and fare or other remedial procedure.Certain embodiments achieve this objective without adding additionalweight to the panel.

In one embodiment, a method comprises processing a phenolic and epoxyresin pre-impregnated composite fabric to bond to a honeycomb core,thereby forming a composite sandwich panel having a smooth, paint-readysurface post-production. The resin system includes a respective curingagent. The fabric reinforcement is ready to lay into a mold without theaddition of additional resin and without the steps required of a typicalhand lay-up. After the prepreg is cured and bonded to the honeycombcore, the prepreg is considered a skin.

In another embodiment, a method is provided for curing a compositepanel, including: inserting a biaxially oriented polypropylene releasefilm and the composite panel into a mold, initiating curing of thecomposite panel at a temperature within a lowermost ten percent of panelcuring temperature range, continuing curing of the composite panel at atemperature above the lowermost ten percent of panel curing temperaturerange, and completing curing of the composite panel at a temperaturewithin the lowermost ten percent of a panel curing temperature range.This procedure has been found to substantially reduce voids and otherdefects and enables the production of a honeycomb panel that is paintready and does not require an intermediate fill and fare step toremediate imperfections in the surface. Alternatively, the process mayproduce a panel which requires substantially reduced defect remediation.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the innovations and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, where:

FIG. 1A is a flow chart of steps of a curing process used to prepare acomposite panel according to an example;

FIG. 1B is a flow chart illustrating a method for assembling a layup ofa set of layers of the composite panel according to an example;

FIGS. 2A and 2B are illustrations of temperature curing profiles of thecuring process used to prepare the composite panel according to anexample; and

FIG. 3 is a block diagram of layers of a composite panel according to anexample.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A curing process is provided for manufacturing a composite panelconfigured to have a smooth, pit-free surface that is ready for paintingwith minimal fill and fair processing. The composite panel can be aphenolic or epoxy resin prepreg honeycomb sandwich panel using a fabricreinforcement. A prepreg is a reinforced material pre-impregnated with apolymer or resin matrix in a controlled ratio.

Examples of honeycomb cores include aramid honeycomb cores, aluminumhoneycomb cores, Nomex® honeycomb cores, and Kevlar® honeycomb cores.Aramid honeycomb cores are frequently used in external vehicular panels(e.g., boat hulls, train cars, auto racing bodies) as well as interiorpanels. Kevlar® aramid honeycomb cores are also available, designed toaerospace standards. Aluminum honeycomb cores have good moisture,corrosion, and fungi resistance as well as flame resistance. They areoften used for floor panels and countertops.

Resins can be both thermoplastic and thermosetting. Types of resinsinclude polymers, phenolic, bismaleimide, cyanate ester, polyester, andvinyl ester. In commercial airplane interior panel design, the resinused is typically an epoxy resin with a built-in curing agent. Epoxyresin, a type of polymer resin, provides the necessary adhesionproperties as well as the viscosity to avoid much resin seepage into thehoneycomb cells. It provides superior strength and dimensional stabilityto many other resins. Vinyl ester resin, a hybrid product, has excellentcorrosion resistance and temperature resistance. However, it is not asstrong as epoxy resin. Polyester resin is relatively inexpensive, easyto use and widely applicable. However, it lacks the strength of theepoxy resin and vinyl ester resin options. Phenolic resin provides heatresistance properties beneficial to flame retardance. Bismaleimide resincan be configured to cure during a two-phase temperature set withoutcreating condensation by-products and can be used for co-curing over ahoneycomb core. Cyanate ester resin has good moisture resistance, makingit a good candidate for panels which are used in hot and dampenvironments. Its electrical properties make it beneficial in reflector,antenna, and signal control uses. The fabric reinforcement can be madefrom different types of fiber including glass, carbon, aramid, andaluminum. Other types of fiber can be used for special applications. Thefibers making the fabric reinforcement can be unidirectional or woveninto different weave types, such as a plain weave, a twill weave, and asatin weave. The fabric reinforcement, for example, may include afiberglass or Kevlar® to be electrostatic charge resistant. Fiberglassis lightweight with moderate tensile strength. Kevlar® is strong and hasgood abrasion resistance. It is often used in panels where impactresistance is important. Carbon fiber is more expensive than fiberglassor Kevlar®, but it has the highest tensile strength. Carbon fiber,additionally, has the highest compressive, bend, and flexural strengthof commonly used reinforcement layer materials. Carbon fiber will oftenbe used in load bearing panels.

There are two main types of manufacturing methods for producing aprepreg; a hot melt method and a solvent dip method. The hot melt methodis conducted in two stages. In a first stage, the resin is heated andcoated onto a paper substrate in a thin film or resin film. In a secondstage, the fabric reinforcement and the resin film are brought together.A roller may be used to apply a pressure to implant the fibers in theresin film, combining heat and pressure. In the solvent dip method, theresin is dissolved in a bath of solvent forming a resin solution. Next,the reinforcing fabric is dipped into the resin solution. Using an oven,the solvent is then evaporated from the prepreg.

In certain embodiments, the curing process is a low cost procedure thatdoes not require any additional materials in the panel to carry out themanufacturing operation, such as a peel ply or surfacing film.

Referring to FIG. 1A, a method 100 is provided for curing a compositepanel having a surface that is smooth, defect-free. At step 110, panellayup is performed to prepare the panel for curing. Layup of the panelshould be performed in a designated controlled contamination area toensure that dirt, dust, aerosols, and other particulate matter do notcontaminate the panel materials. In one embodiment, the layup, pressingand/or curing steps are performed in an environment that complies withUS FED STD 209E cleanroom standards, which are incorporated herein byreference. In another embodiment, the layup, pressing and/or curingsteps are performed in an environment that complies with ISO 14644-1cleanroom standards cleanroom standards, which are incorporated hereinby reference. Us of ISO compliant clean rooms have the advantage thatthey, in some embodiments, have lower particle counts and and smallerparticles. In a preferred embodiment, the clean room is has an ISOclassification of 1-5. In yet another embodiment, the layup, pressingand/or curing steps are performed in an environment that complies withBS 5295 or GMP EU cleanroom standards cleanroom standards, which areincorporated herein by reference. BS 5295 cleanrooms have the advantagethat they do not permit any particles greater than 5 μm.

In preferred embodiments, the count of particles per square meterexceeding 1 μm is 1000 or less, 100 or less, or most preferably 10 orless. In preferred embodiments, the count of particles per square meterexceeding 5 μm is 10 or less, most preferably 1 or less.

Use of the foregoing clean room environments has been consideredundesirable in the industry because of a perception that they wouldincrease the net cost and complexity of the manufacturing process.However, the inventors have found that the techniques described hereingenerate cost savings which substantially outweigh the incrementaladditional cost of performing the process in a clean room environment.

The curing of the composite panel (steps 130-140) can be done in an ovensuch as an autoclave. In an example, the autoclave can have memory andcan be configured to be programmable with a curing temperature profile200 a-b such as shown in FIGS. 2A and 2B. These figures will bediscussed in further detail below.

FIG. 1B is a flow chart that illustrates a method 160 for assembling alayup of the layers of the composite panel, (step 110). The method 160,for example, may be used to manufacture a composite panel as illustratedin FIG. 3.

In some implementations, the method 160 begins with placing at least onerelease paper upon a lower caul plate of a caul press (161). A firstrelease paper, for example, may be positioned with its release sidefacing the caul plate. The release paper (film or other material) isdesigned to help separate the composite panel from the caul press aftercuring. A second release paper may be positioned with its release sidefacing away from the caul plate.

In some embodiments, a caul sheet is placed between the caul plate andthe first release paper. A caul sheet is typically a smooth, flat metalsheet that is used as a surface for laying up panels. In preparation,resin buildup on the caul sheet from previous cures should be removedfrom the caul sheet. The caul sheet can be cleaned using a solvent. Itis important for the caul sheet to be clean and to be free fromscratches, waviness, dents and other imperfections. Imperfections in thecaul sheet are likely to result in imperfections in the cured panel.

In some implementations, at least one prepreg sheet is placed upon therelease paper (162). If two release papers are used, the first prepregsheet is placed upon the release side of the second release paper.

In some embodiments, subsequent prepreg sheets or other material layersare laid either prior to the first prepreg sheet or upon the firstprepreg sheet. For example, a copper foil layer may be applied betweenthe second release paper and the first prepreg sheet. Where multipleprepreg sheets are used, the prepreg sheets may be designed such that aresin differential will exist between the caul plate-facing side of thefirst prepreg sheet and the caul plate-opposing side of the last prepregsheet.

In some implementations, a geometric lattice core sheet is placed uponthe last prepreg sheet (163). The geometric lattice core sheet, forexample, is laid upon the heavier resin application side of the lastprepreg sheet where the prepreg sheet is prepared in the mannerdescribed in U.S. application Ser. No. 14/631,770, filed Feb. 25, 2015,and entitled Composite Sandwich Panel with Differential Resin Layers, orU.S. application Ser. No. ______, filed Aug. 27, 2016 and entitledComposite Sandwich Panel with Differential Resin Layers, the entirety ofwhich are herein incorporated by reference. The geometric lattice coresheet, in a particular example, is a hexagonal honeycomb core.

In some implementations, at least one-prepreg sheet is placed upon thegeometric lattice core sheet with its heavier resin application sidefacing the geometric lattice core sheet (164). The prepreg sheet may beof the same or different material and/or construction than the prepregsheet used to interface with the other side of the geometric latticecore. In some embodiments, subsequent prepreg sheets or other materiallayers are laid upon the first prepreg sheet. Where multiple prepregsheets are used, the prepreg sheets may be designed such that a resindifferential will exist between the core-facing side of the firstprepreg sheet and the core-opposing side of the last prepreg sheet.

In some implementations, at least one release paper is positioned uponthe last prepreg sheet (165). A first release paper, for example, may bepositioned with its release side facing the last prepreg sheet. A secondrelease paper may be positioned with its release side facing away fromthe last prepreg sheet (e.g., towards an upper caul plate). In someembodiments, a caul sheet is placed between the upper caul plate and thesecond release paper.

In an example, the composite panel can be prepared using a layupassembly 300 with the following configuration, whose layers are shown inFIG. 3: a mold 310, a release film 320, a prepreg 330, a honeycomb core340, a prepreg 350, a release film 360, and a mold 370.

For a first layer in the panel layup, a mold 310 is prepared. The mold310 can be a caul sheet or caul plate. A caul sheet is typically asmooth, flat metal sheet that is used as a surface for laying up panels.In preparation, resin buildup on the caul sheet from previous curesshould be removed from the caul sheet. The caul sheet can be cleanedusing a solvent and a cloth. It is important for the caul sheet to beclean and to be free from scratches, waviness, and dents. Imperfectionsin the caul sheet are likely to result in imperfections in the curedpanel.

In an example, the surface of the caul sheet can first be made smooth bya multi-step sanding process. An initial sanding step can be performedfor removing any scratches, resin buildup, and uneven surfaces in thecaul sheet. This sanding step can be performed using a first gritsandpaper having a first grit grade, for example, 100 grit sandpaper. Anadditional sanding step can be performed using a second grit sandpaperhaving a second grit grade, for example, 320 grit sandpaper. Theadditional sanding step is configured to smooth out the caul sheet to afine, even surface. Once the conditioning of the caul sheet is complete,the smoothed caul sheet surface can be wiped with a solvent prior to useto remove any particulates and dust from the caul sheet surface, asdiscussed above.

For a second layer in the panel layup, a release film 320 is placed ontothe mold. The release film 320 or material is configured to helpseparate the prepreg 330 from the mold 310.

In an example, the release film 320 can be made from a polypropylenefilm. Polypropylene is a thermoplastic polymer with resistance tocorrosion and chemical leaching and physical strength and rigidity thatcan withstand the heat of an autoclave.

In an example, the release film 320 can be made from a bi-axiallyoriented polypropylene film (BoPP). BoPP is a material with goodclarity, resistance to UV light, excellent chemical and abrasionresistance, a water vapor barrier, stiffness, dimensional stability, andmost notably an ultra-smooth surface. BoPP also has reasonable scuffresistance and yet is softer and more flexible than polyester or othersimilar films. Further, BoPP film can be flame or corona treatedin-line, for ease of printing, metallizing, or laminating to othersubstrates. When used as release film 320 in the manufacture of aprepreg panel, the BoPP does not compress like typical release papers,and therefore minimizes core telegraphing in the finished panel. Anexample of a biaxially oriented film is available from ViAm Films ofMorristown, Tenn.

During layup, the release film 320 should be positioned such that nocreases or folds are present, because creases and folds will transfer tothe cured panel. Similar to the mold 310, release film 320 should befree of dust and particles, which are also likely to result inimperfections in the cured panel. In an example, the release film 320 isconfigured to release only on one side of the release film and,therefore, is placed with the releasing side facing the prepreg.

For a third layer in the panel layup, a prepreg 330 is positioned. Ifthe prepreg 330 is a prepreg with differential resin content, the higherresin side of the prepreg 330 is to be placed against the core 340. Inan example, prepreg 330 represents a number of prepregs. The prepregscan be positioned in an alternating 90°-0° perpendicular orientations.Alternatively, the prepregs can be positioned at angles 30, 45 or 60degrees relative to the adjacent layer. Prepreg 330 should be kept freefrom contamination during handling, which is facilitated by use of theclean rooms described above.

For a fourth layer in the panel layup, a honeycomb core 340 is preparedand positioned. The surface of core 340 should be substantially flatwith no waviness, and prior to layup, the core 340 should be dried toremove all moisture that may lead to corrosion or degradation. Dryingmay be performed in an oven of approximately 250±50° F., forapproximately one hour, up to twelve hours. Layup of the core 340 shouldbe timely performed after drying. The core 340 can be positioned on theprepreg 330 such that the ribbon direction of the core 340 is parallelto the direction of the fibers of the innermost prepreg 330.

The remaining layup layers, prepreg 350, release film 360, and mold 370,are mirrored and substantially similar to prepreg 330, release film 320,and mold 310, described above.

At step 120, the layup assembly 300 is placed in a press, such as amulti opening press. Between the platens of the multi opening press andthe layup assembly 300, an intermediate padding material such aschipboard or the like may be used to assist in distributing pressureevenly. The press is closed with a pressure of approximately 20-80 psi,more preferably 30-70 psi and most preferably 50-60 psi. If multiplelayup assemblies are to be cured, it is recommended that the layupassemblies be positioned in a single layer (i.e., not stacked).

To cure the panel materials in the layup assembly 300, anin-cold/out-cold process is to be used, an example of which is shown asthe curing temperature profiles 200 a and 200 b in FIGS. 2A and 2B. Inthe in-cold/out-cold process, temperatures are maintained at the low endof the material's cure temperature range, preventing thermal shock tothe panel and avoiding surface defects. Temperatures during the curingprocess may be observed, for example, via a thermocouple (not shown)attached to the panel.

At step 130, curing is initiated at a low end of a cure temperaturerange CT1 of the panel materials (see 210 in FIGS. 2A and 2B). In anexample, temperature CT1 is within the lowermost ten percent of thepanel curing temperature range. Temperature CT1 may be, for example,approximately 115-175° F., preferably 125-165° F., more preferably135-155° F., and most preferably 140-150° F. for a fiberglass phenolicNomex® core sandwich panel. In various embodiments, CT1 is within thelowest 30% of the panel curing temperature range, more preferably in thelowest 20% of the panel curing temperature range, even more preferablyin the lowest 15% of the panel curing temperature range and mostpreferably in the lowest 10% of the panel curing temperature range.

In an example, at step 130, optionally, the temperature is ramped up tothe low end of a cure temperature range CT1 of the panel materials froma lower temperature CT0 (see 260 in FIG. 2B). Given that the panelmaterials may be stored under different conditions and have differentthermal properties, this initial ramp up can minimize an initial thermalshock from any initial temperature differences in the panel materials.

Curing proceeds by ramping up the temperature to a cure temperature CT2(see 220 in FIGS. 2A and 2B). In an example, ramping up of thetemperature can be configured to allow volatiles to escape and to notcause thermal shock, thus leading to a smoother resulting surface. Forexample, the temperature ramp rate should not exceed a rate ofapproximately 10° F./min. In an example, the duration of the ramp up canbe based on a gel time of the resin. Examples of the gel time can varyfrom 30 min to 2 hours.

At step 140, the panel continues curing at temperature CT2 (see 230 inFIGS. 2A and 2B). In an example, temperature CT2 may be between 260° and350° F. At a temperature of approximately 260° F., curing may continuefor approximately 60-90 minutes, for example. CT2 and the cure times atthat temperature may be 200-400° F. and 30-120 minutes, more preferably250-300° F. and 40-100 minutes, or most preferably 250-270° F. at 60-90minutes for a fiberglass phenolic Nomex® core sandwich panel. In variousembodiments, CT2 is within the highest 30% of the panel curingtemperature range, more preferably in the highest 20% of the panelcuring temperature range, more preferably in the highest 15% of thepanel curing temperature range, and most preferably in the highest 10%of the panel curing temperature range.

At step 150, curing is completed at temperature CT1 (see 250 in FIGS. 2Aand 2B). In an example, ramping down of the temperature (see 240 inFIGS. 2A and 2B) can be based on the panel materials and configured notto cause thermal shock. For example, the temperature ramp down may beset so as not exceed a rate of approximately 10° F./min. In otherembodiments, the temperature ramp is in the range of 1-50° F./min, morepreferably 5-30° F./min, and even more preferably 5-20° F./min.

In other embodiments, the process of FIGS. 2A-2B may be modified from acold-in/cold-out process. The process may be a cold-in/warm-out processthat ramps down to temperature CT3 rather than CT1, wherein CT3 isbetween CT1 and CT2. This may be done in recognition of the fact thatthe curing process is mostly complete and the formation of bubbles andother defects are less likely at the tail end of the curing process forcertain epoxy resin formulations. This alternative process is fasterbecause the panels is maintained at a higher average temperature.

Alternatively, the process may be warm-in/cold-out to reduce processingtimes for resins which are more susceptible to defection formationtoward the end of the curing process. In such a process the curing isstarted at CT4 rather than CT1, wherein CT4 is between CT1 and CT2. Thisalternative may be appropriate for resins which outgas substantially atnear the end of the curing time and for resins for which cross linkingoccurs exponentially over time, causing the resin to remain relativelysoft and prone to defect formation until the end of the cure time.

Still further, the process may be modified such at it is hot-in/cold-outor cold-in/hot-out. These are extensions of the previous twoalternatives in that the panel is subjected to the high curingtemperature (CT2) instead of the warm temperature (CT3 or CT4). Thisfurther expedites the curing process and may be the preferred processfor resins having the characteristics described in the preceding twoparagraphs.

The process may be further modified such that the panel is notmaintained at lower (“cold” or “warm”) temperature but rather thermalshock is avoided by ramping the temperature gradually up fromCT1/CT3/CT4 to CT2 and/or gradually ramping the temperature down fromCT2 to CT1/CT3/CT4. The ramps can be exponential or linear to minimizethe processing time as necessary for a given epoxy resin formulation.

In each of the foregoing alternatives, the ramp-up may be of a differentduration than the ramp down. A comparatively short ramp-up time ispreferred for resins which out-gas and/or cross link more intensively atthe end of the curing cycle. A comparatively short ramp-down time ispreferred for resins which out-gas and/or cross link more intensively atthe beginning of the curing cycle.

The foregoing steps yield a honeycomb sandwich panel with a paint readysurface, eliminating the need for fill and fare or film applicationoperations, and in some cases the need for priming the surface, prior topainting. The porosity of prior art panels, conversely, creates the needfor fill and fair or film application, commonly followed by sanding,applying primer, and finally painting. These improvements can save costsassociated with the post-production processing and speed delivery of thepanels to the customer. Because honeycomb sandwich panels are usedthroughout an aircraft (such as monuments, partitions, ceiling panels,cabinetry, galleys, lavatories, luggage bins, etc.) and because everysurface that is not hidden or obscured will generally undergo a paintingprocedure, significant savings in time and costs are realized byproducing a paint ready surface in the curing process.

The foregoing detailed description of the innovations included herein isnot intended to be limited to any specific figure or describedembodiment. One of ordinary skill would readily envision numerousmodifications and variations of the foregoing examples, and the scope ofthe present disclosure is intended to encompass all such modificationsand variations. Accordingly, the scope of the claims presented isproperly measured by the words of the appended claims using theirordinary meanings, consistent with the descriptions and depictionsherein.

1. A method for curing a composite panel to produce a smooth surface,comprising: constructing a panel layup assembly upon a mold, the panellayup assembly including the composite panel, wherein the compositepanel comprises at least a core and a resin formulation, and a releasefilm between the mold and the composite panel, wherein a smooth releasesurface of the release film is in contact with the composite panel uponconstruction; initiating curing of the composite panel at a firsttemperature within a lowermost ten percent of a curing temperature rangeof the resin formulation; continuing curing of the composite panel at asecond temperature above the lowermost ten percent of the curingtemperature range; and completing curing of the composite panel at athird temperature below the second temperature.
 2. The method of claim1, wherein the release film is a polypropylene release film.
 3. Themethod of claim 2, wherein the release film is a bi-axially orientedpolypropylene release film.
 4. The method of claim 1, further comprisinginitiating, prior to the initiating curing of the composite panel at thefirst temperature, heating of the composite panel at a temperature belowthe first temperature.
 5. The method of claim 1, wherein the thirdtemperature is within the lowermost ten percent of the curingtemperature range.
 6. The method of claim 1, wherein the resinformulation is pre-impregnated within a fabric reinforcement.
 7. Themethod of claim 1, wherein the first temperature and the thirdtemperature are less than approximately 150° F.
 8. The method of claim7, wherein the second temperature is approximately 200-350° F. and thecomposite panel is cured at the second temperature for approximately60-90 minutes.
 9. The method of claim 8, wherein a temperature ramp uprate from the first temperature to the second temperature is less thanapproximately 15° F. per minute.
 10. The method of claim 9, wherein atemperature ramp down rate from the second temperature to the thirdtemperature is less than approximately 15° F. per minute.
 11. The methodof claim 1, wherein the smooth surface is paint-ready without need forapplication of a primer.
 12. A smooth-surfaced composite panel,comprising: a core layer; at least one layer of reinforcement material;and a paint-ready exterior surface created through curing a resinformulation component of the composite panel by constructing a panellayup assembly upon a mold, the panel layup assembly including the corelayer, the at least one layer of reinforcement material, the resinformulation, and a release film, wherein the release film is positionedbetween the mold and the composite panel, wherein a smooth releasesurface of the release film is in contact with the composite panel uponconstruction; initiating curing of the composite panel at a firsttemperature within a lowermost ten percent of a curing temperature rangeof the resin formulation; continuing curing of the composite panel at asecond temperature above the lowermost ten percent of the curingtemperature range; and completing curing of the composite panel at athird temperature below the second temperature.
 13. The composite panelof claim 12, wherein, prior to curing, a prepreg layer of the panellayup assembly comprises a) at least a first layer of the at least onelayer of reinforcement material, and b) the resin formulation.
 14. Thecomposite panel of claim 12, wherein the third temperature is within thelowermost ten percent of the curing temperature range.
 15. The compositepanel of claim 12, the paint-ready exterior surface created throughcuring a resin formulation component of the composite panel further byinitiating, prior to the initiating curing of the composite panel at thefirst temperature, heating of the composite panel at a temperature belowthe first temperature.
 16. The composite panel of claim 12, wherein thefabric reinforcement layer has unidirectional fibers.
 17. The compositepanel of claim 12, wherein curing the resin formulation component of thecomposite panel further comprises ramping up the temperature from thefirst temperature to the second temperature at a ramp up rate less thanapproximately 10° F./min.
 18. The composite panel of claim 17, whereincuring the resin formulation component of the composite panel furthercomprises ramping down the temperature from the second temperature tothe third temperature at a ramp down rate less than approximately 15°F./min.
 19. The composite panel of claim 12, wherein the release film isa polypropylene release film.
 20. The composite panel of claim 19,wherein the release film is a bi-axially oriented polypropylene releasefilm.